If you explode a fusion device underground. (pure fusion device without the fission part)
The heat (energy) released will heat up the surrounding rock a bit.
After a while you explode a 2nd device and it will heat-up the rock future.
If you repeat the process...will it able to heat-up the underground rock to (say) 300-500 degree Celcius?

And than you can use the underground rock's heat to produce steam and generate electricity just like a geothermal power plant.

Normal Geothermal power plant need to drill a very deep well (at least 4KM)
perhaps the fusion-geothermal method does not need to drill a very deep well?

Drilling deep is very expensive and that increase the cost of the electricity production.

As stated in wikipedia
"Geothermal power requires no fuel (except for pumps), and is therefore immune to fuel cost fluctuations, but capital costs are significant. Drilling accounts for over half the costs, and exploration of deep resources entails significant risks. A typical well doublet (extraction and injection wells) in Nevada can support 4.5 megawatts (MW) and costs about $10 million to drill, with a 20% failure rate."

The reason to explode fusion devices underground to heat-up the rock is so that you don't need to drill too deep and thus reduce the capital cost and thus reduce the cost of electricity production. (assume we have the pure fusion device and if it works)

Drilling is very expensive but I think you will find that detonating a nuclear device below ground beyond being illegal would be several orders of magnitude more expensive. As Guido said the only method we now possess to create a fusion explosion is by means of a fission device. We are more likely to develope a fusion reactor on the surface than a means to create a fusion explosion without a fission trigger.

Drilling deep is very expensive and that increase the cost of the electricity production.

As stated in wikipedia
"Geothermal power requires no fuel (except for pumps), and is therefore immune to fuel cost fluctuations, but capital costs are significant. Drilling accounts for over half the costs, and exploration of deep resources entails significant risks. A typical well doublet (extraction and injection wells) in Nevada can support 4.5 megawatts (MW) and costs about $10 million to drill, with a 20% failure rate."

The reason to explode fusion devices underground to heat-up the rock is so that you don't need to drill too deep and thus reduce the capital cost and thus reduce the cost of electricity production. (assume we have the pure fusion device and if it works)

Bob DeWoody

My motto: Never do today what you can put off until tomorrow as it may not be required. This no longer applies in light of current events.

The original poster was looking for a device that does not use a fission triggering device. There are none. Back in the 1950s and 60s numerous H-bombs were detonated below ground so there is probably data available on residual heat. One reason underground testing was stopped was the concern over whether large blasts could or would trigger damaging earthquakes. I honestly think using H-bombs to generate electric power for daily use is impractical on many levels.Bob DeWoody

My motto: Never do today what you can put off until tomorrow as it may not be required. This no longer applies in light of current events.

It is analogy to petrol internal combustion engine.
In petrol engine, mixture of petrol and air ignite a small explosion inside cylinder and that pushes the piston.
And this small explosion is repeated at rate of many time per second and that provide lots of power to move the car.

In Laser fusion, they need to ignite the tiny deuterium and tritium pellet at rate of about 10 pellet per second.
And the heat (energy) release will heat up the chamber wall, than the heat can be use to produce steam to run turbine to generate electricity.

My view is: If the ignition of the pellet can be done underground than the size of the pellet can be bigger (can be easily variable from say 100kg t0 10,000kg TNT) without the need of a strong big chamber and the rate of ignition can be lower (maybe one per few hours).
But how to channel the laser beam into underground? Maybe by using fibre optics? (Science fiction?)

The Sedan test at 194 meters (636 ft) underground is of course too shallow compare to geothermal well which can be as deep as 3000 meters.

Anyway all fusion or fission reactors produce heat which must be used to heat water, produce vapor and turn turbines to actuate electrical generators. So they are subject to the laws of thermodynamics as any coal fueled system. There was a cartoon of two engineers watching a huge boiling water reactor and saying: there must be a simpler way of boiling water.
Tullio

Cheng Fan you still need to introduce water into the mix to generate useable energy. Whether the chamber is a man made sphere or a blast created chamber underground, setting off the fusion reaction is only one step in the process which like the man said is only a means to boil water, make steam and spin a turbine.Bob DeWoody

My motto: Never do today what you can put off until tomorrow as it may not be required. This no longer applies in light of current events.

(Note: this is just an imagination)
After many-many continuous small blast (tiny blast actually compare to the fission+fusion one)
than stop the blast.
Then after this use geothermal power plant method to generate electricity. The stored energy underground will be able to run the geothermal power plant for several months (I guess)

After several months when the underground energy depleted than repeat the blast again.

Imagine you have two area A and B. You just have to switch between A an B.
When geothermal power plant is running on stored energy on area A, the blast process is on area B.
When energy A depleted and energy B increased than make a switch, and
now geothermal power plant running on stored energy on area B and the blast process on area A.